With the help of an example differentiate between incomplete dominance and co-dominance.

NCERT Exemplar · Q47

Q. With the help of an example differentiate between incomplete dominance and co-dominance.

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Concept understanding — Mendelian Genetics Basics

Mendelian Genetics Basics

Imagine you have a box of coloured beads — red and white. If you pick one bead from the box, you get either red or white. Now imagine that the colour of your eyes, or the shape of your earlobe, is decided by something like that: a tiny "packet" inside your cells that comes in two versions, and you inherit one from each parent. That is the core idea of Mendelian genetics.

The everyday intuition

You have probably noticed that children often look like their parents — same hair colour, same dimples, same height. But they are never exact copies. Why? Because each parent contributes half of the "instructions" for building a child. Those instructions come in pairs, one from mother and one from father. Sometimes one instruction overrides the other; sometimes they blend. Gregor Mendel, a 19th-century monk, figured out the rules by watching pea plants — tall vs short, yellow vs green seeds — and counting what appeared in the next generation.

The precise meaning

Mendelian genetics is the study of how traits are passed from parents to offspring through genes. A gene is a unit of heredity — a stretch of DNA that codes for a specific characteristic, like flower colour. Each gene comes in different versions called alleles. For every gene, you inherit two alleles: one from your mother, one from your father.

If the two alleles are identical, you are homozygous for that trait. If they are different, you are heterozygous. In a heterozygous pair, one allele may be dominant — it shows up in the appearance — and the other recessive — it stays hidden unless both alleles are recessive.

Note

Mendel's key insight was that traits are not blended like paint. Instead, alleles remain separate and are passed on intact. A recessive allele can skip a generation and reappear later, unchanged.

Why it matters

Mendelian genetics is the foundation of modern biology. It explains:

Why some diseases run in families (like cystic fibrosis or sickle-cell anaemia)
How plant and animal breeders create new varieties
Why you might have your grandmother's eyes but not your mother's

The NCERT textbook states that Mendel's work established the laws of inheritance — the Law of Dominance, the Law of Segregation, and the Law of Independent Assortment. These laws describe how alleles separate during the formation of eggs and sperm, and how different genes are inherited independently of one another.

Key terms at a glance

Gene: a unit of heredity on a chromosome
Allele: a variant form of a gene
Dominant: the allele that expresses itself even when paired with a different allele
Recessive: the allele that expresses itself only when paired with an identical recessive allele
Homozygous: having two identical alleles for a gene
Heterozygous: having two different alleles for a gene …

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Incomplete dominance produces a blended intermediate phenotype in heterozygotes (e.g., pink flowers from red × white), while co-dominance expresses both alleles equally and simultaneously without blending (e.g., AB blood type showing both A and B antigens).

When Mendel worked with pea plants, he observed traits that followed a clear dominant-recessive pattern: tall always masked dwarf, yellow always masked green. But nature doesn't always play by these rules. Two important exceptions to Mendelian dominance are incomplete dominance and co-dominance, and understanding the difference between them reveals how gene expression can be far more nuanced than a simple "winner takes all" scenario.

Incomplete Dominance: The Blending Story

Incomplete dominance occurs when neither allele is fully dominant over the other, and the heterozygous condition produces a phenotype that is intermediate between the two homozygous parents. The classic example comes from snapdragon flowers (Antirrhinum).

When you cross a pure-breeding red-flowered snapdragon (RR) with a pure-breeding white-flowered one (rr), the F₁ generation doesn't show red flowers (as you'd expect if red were dominant). Instead, all F₁ plants produce pink flowers (Rr). The red pigment is diluted, creating an entirely new phenotype that sits halfway between the parents.

If you self-pollinate these pink F₁ plants, the F₂ generation shows a 1:2:1 ratio:

1 Red (RR)
2 Pink (Rr)
1 White (rr)

Notice that the phenotypic ratio matches the genotypic ratio here, because the heterozygote is visibly different from both homozygotes. The key idea is blending: the two alleles mix their effects to produce something in between.

Note

Another example is the Mirabilis jalapa (four o'clock plant), which also shows red, pink, and white flowers following the same inheritance pattern.

Co-dominance: Both Alleles Speak Up

Co-dominance is fundamentally different. Here, both alleles in the heterozygote are fully expressed simultaneously, without any blending. Each allele produces its own distinct, detectable product, and both are visible in the phenotype.

The textbook example is human ABO blood groups, specifically the AB blood type. The gene for ABO blood type has three alleles: I^A, I^B, and i. The I^A allele codes for A antigens on red blood cells, while I^B codes for B antigens.

When someone inherits both I^A and I^B (genotype I^A I^B), they have AB blood type. Critically, their red blood cells carry both A and B antigens on their surface. There's no blending into some intermediate antigen; both are present and fully functional. You can detect both with specific antibody tests. …

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Q.With the help of an example differentiate between incomplete dominance and co-dominance.

Incomplete Dominance: The Blending Story

Co-dominance: Both Alleles Speak Up

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